The present invention relates to a method for the preparation of an article comprising a substrate containing silicon and a barium-strontium aluminosilicate barrier layer which functions as a protective environmental/thermal barrier coating.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures in aqueous environments. By aqueous environment is meant a water and/or steam environment. It has been found that these silicon containing substrates can recede and lose mass when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200xc2x0 C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x. It has been found that barium-strontium aluminosilicate barrier layer as a top layer is effective to reduce formation of volatile silicon species and thus reduce recession and mass loss. However, the barrier layer has been found to exhibit cracks in the fabricated matrix. Naturally it would be highly desirable to provide an external barrier coating for silicon containing substrates which is substantially crack free (cracks through the barrier layer to the substrate) and which would inhibit the formation of volatile silicon species, Si(OH)x and SiO, and thereby reduce recession and mass loss.
Accordingly, it is the principle object of the present invention to provide method for the preparation of an article comprising a silicon containing substrate with a barium-strontium aluminosilicate (BSAS) barrier layer which is substantially crack free and which inhibits the formation of gaseous species of Si, particularly Si(OH)x, when the article is exposed to a high temperature, aqueous environment.
It is a further object of the present invention to provide a method for producing an article as aforesaid.
The invention relates to a method for producing an article comprising a silicon containing substrate and a barium-strontium aluminosilicate barrier layer which is substantially crack free and which inhibits the formation of gaseous species of silicon and/or provides thermal protection when the article is exposed to a high temperature, aqueous environment as defined above. In accordance with the present invention, the method comprises heating the silicon containing substrate to a temperature (T) of about greater than or equal to 1100xc2x0 C., applying the barium-strontium aluminosilicate barrier layer to the substrate while the substrate is held at a temperature (T) of at least about 1100xc2x0 C. and thereafter holding the coated substrate at the temperature (T) for a minimum of at least about 15 minutes prior to cooling down the coated substrate to room temperature. The resulting article exhibits a substantially crack-free barrier layer having a crystalline structure of at least 80% by volume composed of celsian and hexacelsian phases. The nature of the crystalline phases depends on the time at temperature at which it is held. The barrier layer has a coefficient of thermal expansion which is compatible with that of the Si containing substrate prior to subsequent heat treatment.
In a preferred embodiment of the present invention the article can include one or more intermediate layers between the silicon based substrate and the barium-strontium aluminosilicate barrier layer. The intermediate layer(s) serve(s) to enhance adherence between the barrier layer and the substrate and/or to prevent reactions between the barrier layer and the substrate.
Further objects and advantages of the present invention will appear hereinbelow from the following detailed description.